1. Technical Field
The present disclosure relates to a method of manufacturing a fluoride phosphor.
2. Background Art
A light emitting diode (LED) is a light emitting device which is frequently used as a substitute for conventional light sources such as an incandescent bulb. Such a light emitting diode is useful as a backlight of display device, a warning lamp, an indicating lamp, and for luminarie. Also various light emitting devices which employ a laser diode (LD) are proposed, in those, in a similar manner as in such a light emitting diode, a laser diode is used in combination with a phosphor. The light emitting diodes and laser diodes are both semiconductor light emitting elements fabricated using an alloy of group III-V compound such as gallium nitride (GaN). Combining such a semiconductor light emitting element and a phosphor, various light emitting devices to emit light of a white color, an incandescent lamp color, or an orange color have been developed. Those light emitting devices to emit a white light etc., can be obtained based on the principle of light-color mixing. As for the system to emit a white light, there are well-known systems such as a system which employs an ultraviolet-light emitting element and three types of phosphors which emit lights of RGB colors respectively, and a system which employs a blue-light emitting element and a yellow-light emitting phosphor. Light emitting devices of the type which employ a blue-light emitting element and a yellow phosphor etc., are in demand in a wide range of fields such as luminaries, e.g., a fluorescent-type lamp, an on-vehicle luminaire, a display, a backlight for liquid crystal. Of those, for the phosphors used for a backlight of liquid crystal, in order to reproduce a wide range of colors on a chromaticity diagram, high color purity is also demanded along with the light emitting efficiency. Particularly, the phosphors used for backlights for liquid crystal are required to have compatibility in combination with a filter, and a phosphor with a narrow half width of the emission peak has been in demand.
For example, as for red phosphors which have an excitation band in blue-color region and a narrow half width of the emission peak, fluoride phosphors having compositions such as K2TiF6:Mn4+, Ba2TiF6:Mn4+, Na2TiF6:Mn4+, and K3ZrF7:Mn4+ etc., are known (for example, see JP 2009-528429A).
As for another phosphor, fluoride phosphor K2SiF6:Mn4+ is also known (for example, see JP 2010-209311A).
As for a yet another phosphor, excitation/emission spectra and mechanism of emission of a fluoride complex phosphor activated with Mn4+ is also known (for example, see A. G. Paulusz, Effective Mn (IV) Emission in Fluoride Coordination; J. Electrochemical Soc., 120 N7, 1973, p942-947).
Such a fluoride phosphor which is activated with tetravalent Mn and can emit red light with a narrow half width of the emission peak, and a light emitting device using the same are assumed to be particularly suitable for the use as a backlight in a liquid crystal device. Thus, practical use of those has been demanded. On the other hand, conventional fluoride phosphors activated with tetravalent Mn have poor resistance to water. That is, Me which are tetravalent Mn present on the surface of the phosphor particles are exposed to the moisture in the air and oxidized to manganese dioxide (MnO2) which has a dark color, which is thought to resulting in degradation of the luminance. Accordingly, satisfactory durability cannot be obtained in a long-term reliability test, so that the usage which requires reliability, for example, for LCD backlight applications and in-vehicle applications, has been regarded difficult to implement.